1. Field of the Invention
The present invention relates to a clock supply apparatus, more particularly a clock supply apparatus which is provided for each of a plurality of offices performing dependent synchronization.
In general, multiplex transmission is used for inter-office transmission so that a large number of channels are transmitted together from one office to another. In this case, it is indispensable to attain synchronization with the other office. As this synchronization system, use has been made of the stuff multiplex system in the past.
Recently, however, there has been a tendency for use of the so-called dependent synchronization system. The present invention assumes the use of this system. In this dependent synchronization system, a master clock from a high precision oscillator installed in the master office is commonly received by all the slave offices. In synchronization with this master clock, transmission processing is performed in the respective slave offices. When doing this, a stuffing operation or a destuffing operation as in the case of the synchronization system according to the above-described stuff multiplex system becomes unnecessary.
2. Description of the Related Art
As will be explained later with reference to the drawings, in a conventional clock supply apparatus, a 0-system system clock and a 1-system system clock always coexist from an upstream side clock receiver unit 14 to the downstream transmission units 13. When the system clock of any of the systems is stopped, this situation is saved by the system clock of the other system without fail, so the reliability of supply of the clock has become extremely high.
However, if a fault occurs in the selection control part of the coexisting system clocks, the coexistence of the two systems of system clocks is of no use at all. That is, the reliability of the entire clock supply apparatus becomes far lower in comparison with the reliability obtained by the coexistence of the 0-system and 1-system system clocks. This is a problem.